To configure the api_key for an API client, which of the following methods must be used?
A) The api_key must be passed directly to the API server with each request.
B) The api_key can be set either by passing it to the client during initialization or by setting the OPENAI_API_KEY environment variable.
C) The api_key should be embedded in the request headers manually for each API call.
D) The api_key must be included as a query parameter in each URL request.